Figure 2.

Noncoding loci at the Xic. On Xi, RepA is thought to recruit PRC2 to the Xist promoter to (paradoxically) upregulate transcription. PRC2 may then be loaded onto Xist, which remains tethered to its allele of origin via YY1 interactions with RNA and DNA. Meanwhile on Xa, Tsix expression is believed to repress Xist through a combination of several mechanisms: titrating away PRC2, preventing its proper docking onto RepA/Xist; recruiting Dnmt3a, laying down DNA methylation (Ⓜ) to silence the Xist promoter; and/or directly base pairing with Xist RNA, becoming a substrate for Dicer-dependent processing into small RNAs.